Silicone elastomers have been employed in sealing applications for many years. For O-rings and other molded seals, high temperature vulcanizable (HTV) compositions are generally used. Such compositions are somewhat less expensive than RTV-2 compositions, and also exhibit somewhat greater thermal stability. These compositions may include polydimethylorganosiloxanes, fillers, and organic peroxides which function to cure the compositions by free radical-induced crosslinking. Pour-in-place and similar gaskets often cannot employ HTV compositions, however, as the parts being sealed, the fluids sealed within them, or both, cannot stand the elevated temperatures required for curing. Examples of HTV elastomers and their components may be found in U.S. Pat. Nos. 4,782,107; 4,728,687; and 5,550,185.
Two-component room temperature vulcanizable compositions ("RTV-2") have been used as elastomeric sealants. Such compositions generally contain unsaturated alkenyl-functional organopolysiloxanes such as those containing vinyl, allyl, acryloxy, methacryloxy, or .omega.-terminal alkenyl radicals such as .omega.-hexenyl radicals, in conjunction with an Si--H functional organopolysiloxane. One or both components also contain a hydrosilylation catalyst. Use of fillers such as fumed silica, quartz flour, calcium carbonate and the like is relatively common. However, such elastomers do not, in general, have the thermal stability of HTV elastomers, and are inconvenient to use because of their two-part formulation. RTV-2 compositions employing other reactive systems are known as well, for example those disclosed in U.S. Pat. No. 4,892,907.
Single component, room temperature vulcanizable ("RTV-1") silicone sealants have been known in the construction trade for many years. Such sealants often include acyloxy-functional silicones as one component of a storage-stable blend which also generally includes a silanol-functional silicone such as an .alpha.,.omega.-dihydroxypolydimethylsiloxane. The composition begins to cure upon exposure to atmospheric moisture. To increase the viscosity and the "body" of the cured elastomer, large amounts of relatively inexpensive fillers such as ground calcium carbonate are incorporated in the sealant composition. Such sealants generally cure relatively slowly and first form a cured skin which impedes ingress of moisture necessary to cure the interior. While useful as caulks in the construction industry, such sealants have little use as sealants for other applications. Moreover, such sealants do not possess exceptional thermal stability, exhibiting more severe degradation than other silicone elastomers at elevated temperatures.
In environments where gasketing material must be cast or poured in place, the ability to use HTV silicone elastomers, as previously indicated, is severely limited. Moreover, such gaskets are often used to seal off cavities or passages which contain fluids, particularly in the automotive sector where gasketing materials may be exposed to water, antifreeze, gasoline, brake fluids, hot and cold oil, automatic transmission fluid, and gear and axle lubricants and fluids. In U.S. Pat. No. 5,013,781, for example, are disclosed RTV-1 compositions containing organopolysiloxane resins composed of M and Q units or of M, D, and Q units, together with an inorganic filler, an alkoxysilane adhesion promoter, and a ketoxime silicone crosslinker. The fillers used may be non-reinforcing or reinforcing silica filers, or may be non-reinforcing, non-silaceous fillers such as calcium carbonate, zinc carbonate, magnesium oxide, aluminum hydroxide, iron oxide, zinc oxide, titanium oxide, and powdered mica. The fillers were viewed as substantially equivalent, with the highest initial properties and greatest retention of properties exhibited by fumed silica and iron oxide.
The substantially equivalent performance of a wide variety of non-reinforcing fillers is generally accepted in the organopolysiloxane art. For example, in U.S. Pat. No. 4,748,166, cited as equivalent fillers, either alone or in admixture, are ground quartz, diatomaceous earth, calcium carbonate, calcined clay, natural titanium dioxide (rutile), oxides of iron, zinc, chromium, zirconium, and magnesium, hydrated and non-hydrated alumina, boron nitride, lithopone, barium metaborate, powdered cork, wood sawdust, inorganic and organic fibers, and the like. Similar shopping lists of fillers may be found in U.S. Pat. Nos. 4,782,107 and 5,268,441, the latter of which discloses treatments suitable for rendering the fillers hydrophobic. Where heat resistance is required, it is generally acknowledged that iron oxide, zirconium oxide, and barium zirconate are good filler candidates. The use of red iron oxide in automotive RTV-2 gaskets is disclosed in U.S. Pat. No. 4,892,907. The most common non-reinforcing filler or extender is calcium carbonate, as disclosed in the Examples in U.S. Pat. Nos. 4,748,166; 4,962,151; 5,118,738; and 5,569,750.
Recently, under pressure to further increase fuel economy, fluids such as those used in axles, differentials, transmissions, and transaxles have come into scrutiny by automobile manufacturers. Use of viscous fluids in many of these applications results in a large energy loss. Moreover, since this energy loss appears in the form of heat, the lifetime of the various fluids is decreased. The current trend in such fluids is therefore to lower viscosity. For example, axle and gear lubricants have previously been typically composed of relatively viscous oily or greasy components such as heavy paraffin hydrotreated distillate, heavy paraffin solvent dewaxed distillate, and solvent dewaxed residual oil, but are now being replaced by lower viscosity lubricants containing numerous synthetic additives to increase the lubricity, necessary due to the lower film forming capacity of such lubricants, particularly at elevated temperature. Examples of these moderate to high pressure lubricant additives are olefin sulfides and organophosphate esters. Unfortunately, it has been found that the organopolysiloxane elastomers previously used with great success when exposed to conventional lubricants exhibited total failure in less than 200 hours operation in simulated use tests when exposed to more aggressive lubricants.
It would be desirable to provide gasketing and sealing materials which retain the superior thermal stability of silicone elastomers yet which can provide durability when exposed to aggressive fluids. It would be further desirable to provide RTV-1 compositions suitable for forming such gaskets and sealants.